Oil and gas accumulations are found at depth in different geological basins worldwide. Exploration and production of such accumulations rely on the construction of a well according to a well plan.
Various well types exist and are defined according to usage such as wildcats or those used in exploration, delineation, and production and injection. Variations in well profile exist also according to vertical, slant, directional and horizontal trajectories. Each well differs according to the oil company's objectives and the challenges that a given basin presents from the surface of the earth or the ocean to reaching the hydrocarbon reservoir at a given underground depth.
Engineering challenges are related to the location of the well-site such as onshore or offshore, seawater depths, formation pressures and temperature gradients, formation stresses and movements and reservoir types such as carbonate or sandstone. To overcome these challenges, a highly detailed well plan is developed which contains the well objective, coordinates, legal, geological, technical and well engineering data and calculations.
The data is used to plot the well profile, and plan its execution using precise bearings, which is designed in consecutive telescopic sections—surface, intermediate and reservoir. To deliver the well objective and maintain the integrity and operating capacity of the well over its lifecycle, a given wellbore with multiple sections and diameters is drilled from surface. Although there are many variants, a simple vertical well design could include the following dimensions: a surface or top-hole diameter of 17½″ (445 mm), intermediate sections of 13⅝″ (360 mm) and 9⅝″ (245 mm) narrowing down to a bottom-hole diameter of 8½″ (216 mm) in the reservoir section.
Each consecutive section is ‘cased’ with a number of metal tubes placed into the wellbore with the specified diameter according to the length of the section. Casing tubes are connected to each other after which they are cemented into the outer wall of the well. In this way, a well is constructed in staged sections, each section dependent on the completion of the previous section until the well is isolated from the formation in question along the entire distance from surface to the reservoir.
Scarcity of oil and gas is driving oil and gas companies to explore and develop reserves in more challenging basins such as those in water-depths exceeding 6,000 ft (1800 m) or below massive salt sections. These wells have highly complex directional trajectories with casing designs including 6 or more well sections. Known in the art as ‘designer’ or ‘close tolerance casing’ wells, these wells have narrow casing diameters with tight tolerances and have created a need to enlarge the wellbore to avoid very narrow reservoir sections and low production rates.
Therefore, the bottom-hole assemblies that are needed to drill these wells routinely include devices to underream the well-bore below a given casing diameter or other restriction. In this way, underreaming has become an integral part of well construction and there is now an increased dependence on underreaming to meet planned wellbore diameters. After underreaming, the underreamer is tripped out of the borehole and replaced by the caliper, which is an instrument for measuring the internal dimensions of the wellbore either mechanically, using extended fingers that contact the wall of the wellbore, or by acoustic techniques using reflected acoustic signals from the wall of the wellbore.
Previously, the underreamer and caliper have been considered as two separate tools, each involved in distinct functions. Typically, an underreaming run could take 24 hours, after which a further 24 hours would be required for preparation of the caliper run. A further 24 hours could be taken in the caliper run before knowledge could be gained of actual wellbore diameters. This time-lag between underreaming and caliper measurements therefore could easily exceed 48 hours depending on the depths involved. If the actual hole diameter did not match the planned diameter, casing tolerances would not be met and a corrective run would be required. Consequently, the whole cycle of underreaming and caliper measurements would need to be repeated.
In other applications such as expandable tubular or increased cementing thicknesses, the tolerances between the enlarged well-bore and the expanded steel tube and cementing thickness are very close. Variations of 1″ (25 mm) in the diameter can lead to the failure of the well construction activity.